Iron nickel chromium alloys

ABSTRACT

IRON-BASE ALLOYS FOR SHEATHING ELECTRIC HEATER ELEMENTS INCLUDE SPECIALLY PROPORTIONED AMOUNTS OF NICKEL (ABOUT 15% TO ABOUT 23%), CHROMIUM (ABOUT 17% TO ABOUT 23%), SILICON (ABOUT 0.3% TO ABOUT 1.5%) AND CERIUM (AN EFFECTIVE AMOUNT UP TO ABOUT 05%) TO ASSURE HIGH OXIDATION RESISTANCE AND GOOD CORROSION RESISTANCE.

United States Patent f 3,729,308 IRON NICKEL CHROMIUM ALLOYS Herbert L.Eiselsteiu and James C. Hosier, Huntington,

W. Va., assignors to The International Nickel Company, Inc., New York,N.Y. No Drawing. Filed July 21, 1970, Ser. No. 56,977 Int. Cl. C22c39/20 US. Cl. 75-128 E Claims ABSTRACT OF THE DISCLOSURE Iron-basealloys for sheathing electric heater elements include speciallyproportioned amounts of nickel (about to about 23%), chromium (about 17%to about 23%), silicon (about 0.3% to about 1.5%) and cerium (aneffective amount up to about .05%) to assure high oxidation resistanceand good corrosion resistance.

This invention is directed to improved oxidation and corrosion resistantlow cost iron-base alloys particularly suitable for use as electricheater element sheathing.

One type of electric heater element comprises a resistance conductorwhich is enclosed in a tubular metal sheath, with the resistanceconductor embedded in and supported in spaced relation to the sheath bya densely compacted layer of refractory, heat-conducting, electricallyinsulating material. The resistance conductor may be a helically woundwire member and the refractory insulating material may be granularmagnesium oxide (Ma A commercial alloy which is currently used assheathing material is an iron-base alloy which nominally contains, byweight, 32.5% nickel, 21% chromium, and small amounts of carbon,manganese, sulfur, silicon, copper, aluminum and titanium. While thiscommercial alloy has performed satisfactorily in the heater sheathingapplication, an alloy exhibiting enhanced resistance to oxidation atlower cost would be a highly desirable material. Moreover, it would alsobe of considerable commercial advantage were these benefits to beobtained without adversely affecting other characteristics for whichsuch prior art alloys are known, notably resistance to stress-corrosioncracking and good weldability. The present invention is thereforeprimarily directed to achieving these overall objectives.

It has now been discovered that specially proportioned,iron-nickel-chromium alloys containing correlated percentages ofchromium, nickel, silicon and advantageously, cerium, exhibitextraordinary resistance to oxidation at elevated temperature and, inaddition, are quite resistant to stress-corrosion cracking as well asbeing readily weldable.

It is an object of this invention to provide an ironbase alloy which isweldable and exhibits a high resistance to cyclic oxidation at elevatedtemperatures and good resistance to stress corrosion cracking.

It is another object of this invention to provide an electric heaterelement which is sheathed in an iron-base alloy characterized byexcellent resistance to cyclic oxidation at elevated temperatures andgood resistance to stress corrosion cracking.

Other objects and advantages will become apparent from the followingdescription.

Generally speaking, the present invention contemplates weldableiron-base alloys composed of, by weight, from about 15% to about 23%nickel, from about 17% to about 23% chromium, from about 0.3% to about1.5% silicon, cerium in a small but effective amount sufiicient toimprove oxidation and corrosion resistance, the cerium being up to about0.05%, up to about 2% manganese, up to about 0.15% carbon, up to about0.5% aluminum,

3,729,308 Patented Apr. 24, 1973 up to about 0.5 titanium, up to about0.015 sulfur and the balance essentially iron. In addition it isimportant that the various constituents of the alloys be correlated soas to satisfy the following relationships which, upon being observed,result in high resistance to cyclic oxidation and good resistance tostress corrosion cracking, respectively:

1178-9986 (percent Ce)1085 (percent Si)-49.78 (percent Cr)+3965 (percentSixpercent Ce)+304.5 (percent Cr percent Ce) +44. 84 (percent SixpercentCr)+16.65 [percent Mn/ (percent Si-l-percent Ce)] must be 560 1156+109.1 (percent Ni)-+2690 (percent Mnxpercent Ce)5.97 (percent Mn percentNi)1.57 (percent Ni percent Cr)+358.5 (percent Si must be 2500 Where theoptimum of properties would not consistently be required, the cerium canbe omitted, but even then it is much preferred to use, at least a smallamount, e.g., 0.005%, for it has been found that in conjunction withsilicon it counteracts the tendency for manganese to detract fromoxidation resistance and resistance to stress corrosion cracking.Accordingly, these elements should be controlled such that the ratiopercent Mn/ (percent Si-i-percent Ce) has a value less than about 0.6and the ratio percent Ni/ percent Cr has a value in the range from 0.9to 1.2. The alloys of the invention exhibit resistance to cyclicoxidation at 1800 F. equal to or superior to that of the sheathingalloys commonly used.

An advantageous composition of the alloy of the invention is composedof, by weight, about 20% nickel, about 20% chromium, about 0.6% silicon,about 0.02% cerium and the balance essentially iron except for smallamounts of incidental elements and impurities including up to about 1%manganese.

In carrying the invention into practice, chromium and nickel promoteresistance to oxidation, as well as resistance to general corrosion andto stress corrosion. Chromium in amounts in excess of about 23% has anadverse effect on workability. In general, the higher the chromiumcontent within the specified range, the better the alloy produced,particularly with respect to oxidation resistance. Amounts of nickelbeyond 23% are unnecessary and should the percentage fall much below 15%various characteristics can be adversely affected. In seeking the bestcombination of results a range of from 19% to 21% or 22% chromium andfrom about 18% to 22.5% nickel is quite satisfactory.

Silicon and cerium both aid in providing the required agree of oxidationand corrosion resistance. Silicon confers improved oxidation resistance;however, an amount of silicon above the range specified leads toproblems in weldability, particularly hot cracking and excessivefluidity. A silicon content of from 0.6% or 0.7% to about 1% or 1.1% isdeemed quite beneficial. Cerium is advantageously present in the alloysin the amount of from about 0.01% to about 0.05%. The element cerium inamounts Within the specified range makes an important contribution tothe oxidation resistance and to stress corrosion resistance. It is goodpractice to include at least 0.01% cerium since the alloys are then lesssensitive in respect of their corrosion resistance to variations in theamounts of the other alloy constituents, particularly, nickel, chromium,silicon and manganese. Quantities of cerium in excess of the specifiedrange lead to difficulties in forging, rolling and in weldability. Thus,a cerium range of from 0.015% to about 0.04% is preferred.

The cerium which is employed in the alloys of this invention isordinarily added as misch metal which also contains lanthanum and otherrare earth elements. Typically, an addition of misch metal is composed,by weight,

of about 50% cerium, 20% lanthanum, with the balance other rare earthelements. It will be understood that in the alloys of this invention,Where additions of cerium are made, there will usually also be severalthousandths of a percent of lanthanum present.

The alloys of the invention may also include relatively small amounts ofother elements which can be present as impurities without substantialdetrimental effect. Such elements are contained in the alloys of theinvention due to their unavoidable presence in the raw materialsemployed, or as a result of their use for one purpose or another duringthe preparation of the alloys. Thus, manganese in the amount of up to2%, by weight, may be tolerated, but it is more advantageous to have nomore than 1% present. Manganese in amounts in excess of the specifiedmaximum has an adverse effect upon oxidation resistance and upon stresscorrosion cracking. Up to about 0.15% by weight of carbon can betolerated, but it is desirable that the alloy contain no more than about0.1%. Excessive carbon may result in precipitation of carbides andhence, brittleness, which gives rise to problems in forming the alloy.It is quite difficult to obtain an alloy of this type which is entirelyfree from sulfur, and in this case up to 0.015% can be tolerated. Copperis also a common impurity which can be tolerated in amounts up to amaximum of 0.5%. Amounts of copper greater than this tend to reduce hightemperature oxidation resistance. Aluminum, titanium and calcium areused as deoxidizers in melting the alloys of the invention, and residualamounts of these elements may remain in the final alloy composition.Thus, up to about 0.5% each, by weight, of aluminum and titanium may bepresent and in some cases, small amounts of calcium have been detected.

For the purpose of giving those skilled in the art a better appreciationof the advantages of the invention, the following illustrative examplesare given.

EXAMPLE A number of heats of the alloys of the invention were preparedby charging appropriate amounts of iron, nickel and chromium into an airinduction furnace and melting down the charge. Just before tapping theheat, the required silicon, aluminum, titanium and cerium additions (thelatter in the form of misch metal) were added, and last of all, calcium.The heat was then poured into molds to produce 4 inch diameter ingots.The ingots thus prepared were forged into A-inch thick flat bars andthen the fiat bars were cold rolled to 0.125 inch thick sheet. Sectionswere cut from the sheet for use as cyclic oxidation specimens and forstandard U-bend stress corrosion cracking test specimens and theremaining sections of the sheet were then welded end to end to make aroll of sheet which was then cold rolled to 0.025 inch thick sheet, thegauge commonly employed in making heater element sheathing. Thefollowing Table I gives the composition of abnumber of alloys which wereprepared as described a ove.

The mechanical properties indicated in the above Table II for the alloysof the invention are quite comparable to the properties of the commonlyemployed prior art sheathing alloy referred to above herein. Such analloy is produced to meet a sheet specification wherein the minimumtensile strength is 75,000 p.s.i., the minimum yield strength (0.2%offset) is 30,000 p.s.i., the minimum elongation is 30% and the maximumhardness is 80 R In the following Table III, the depth of the oxidationattack on alloy sheet having a thickness of 0.125 inch and the weightloss sustained thereby is set forth under cyclic oxidation conditionswherein a test cycle was employed in which samples were heated to atemperature of 1800" F. in a furnace in air for 15 minutes, followed by5 minutes cooling in air outside the furnace, such cycles being repeatedfor a period of 1,000 hours.

TABLE III.OXIDAIION ATTACK Depth of Weight loss, attack, in. mgJem.

Alloy No.

Original gaugeRemaining good metal Depth of attack= It will be observedfrom the above table that the alloys of the present invention, after1,000 hours of test, have sustained relatively slight damage fromoxidation attack. A commercial alloy of the composition described aboveand tested under the same conditions sustained oxidation damage to adepth of about 0.008 inch and a weight loss of over 120 mg./cm. Alloyswhich sustain a weight loss of up to 60 mg./cm. in this oxidation testare considered to have exhibited high resistance to cyclic oxidation.

The alloys of Table I were also tested for resistance to stresscorrosion cracking. Standard restrained U-bend stress corrosion crackingtest specimens were prepared from alloy pieces which had the dimensions6" x 0.5" x 0.125". The restrained test specimens were immersed in aboiling concentrated magnesium chloride solution and were periodicallyexamined for cracking. When the tests were terminated after 720 hours(30 days) none TABLE I.OHEMICAL ANALYSIS OF ALLOYS 3 0. C Mn Fe S S1 CuNi Cr Al Ti Ce 0. 05 0. 41 Bal. 0.009 0.90 0. 02 22. 14 18.62 0.31 0.020. 05 1. 10 Bal- 0.005 0. 71 0. 02 20. 27 20. 37 0. 11 0. 12 0. 04: 0.08 Bal. 0. 007 0. 81 0. 09 20. 69 19. 64 0. 11 0. 07 0. 09 0. 36 Bel. 0.008 0. 93 O. 02 18. Q2 17. 63 0.38 0.22 0.11 1. 63 Bal. 0. 008 0. 84 0.02 18. 2 0 17. 19 0.022 0. l4 0. 09 O. 41 Bal. 0. 008 1.03 0.02 22. 0622.72 0. 004 0. 19 0.09 1. 80 Bal. 0. 006 0. 94 0.02 22. 21 23.01 0. 170. 15

N orE.Bal.=balance iron plus impurities,

The tensile properties and hardness at room temperature of the alloys ofTable I are set forth in the following Table II.

of the alloy specimens representing the alloys of Table I had failedwhereas the commercial alloy mentioned above failed in just over 300hours. It is considered that alloys subjected to this test which survive500 hours have exhibited good resistance to stress corrosion cracking.In this connection it is most advantageous that the ratio of nickel tochromium be at least unity, the silicon content should be 1% or more andthe manganese should not exceed 0.5

The alloys of Table I were further tested for weldability by preparingautogenous TIG Welds and examining the welds for cracking. Theweldability of the alloys ranged from good to excellent.

A heat of commercial size has been produced having mechanical propertiescomparable to those set forth in Table II. This heat was made in afurnace having a nominal capacity of 5,000 pounds. The heat producedweighed roughly 5,500 pounds and had the following composition expressedin weight percent:

Nickel 20.32 Chromium 19.94 Silicon 0.60 Cerium 0.022 Manganese 0.31Carbon 0.04 Sulfur 0.007 Phosphorus 0.010 Aluminum 0.17 Titanium 0.17Calcium 0.001 Copper 0.03 Iron Balance After forging, the heat wasrolled to hot band A-inch thick, and thereafter cold rolled to thedesired gauge of 0.090 inch. Cyclic oxidation and stress corrosioncracking test specimens were prepared and tested. The alloy exhibited aweight loss of only 3 mg./cm. after 1000 hours of the cyclic oxidationtest and standard U-bend specimens for the stress corrosion crackingtest were exposed to test conditions for 720 hours without failure.Accordingly, this alloy clearly exhibits high resistance to cyclicoxidation and good resistance to stress corrosion cracking.

The alloys of the present invention also exhibit a resistance to generalcorrosion similar to that of Type 304 stainless steel. This property isof some importance in view of the fact that in normal use as sheathingalloys, they are often exposed to contact with various solutions atelevated temperature.

There has thus been disclosed a family of alloys having surprisinglygood resistance to oxidation at elevated temperatures, though containingnickel in substantially smaller amounts than has been the practice inthe art, particularly in the field of sheathing alloys for electricalheater elements. These alloys may also be usefully em ployed as heatexchangers and process piping, carburizing fixtures and retorts andfurnace components.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

We claim:

1. A weldable, oxidation and corrosion-resistant alloy consistingessentially, by weight, of from about 15% to about 23% nickel, fromabout 17% to about 23% chromium, from about 0.3% to about 1.5% silicon,from a small but effective amount of cerium up to about 0.05 to improveoxidation and corrosion resistance, up to about 2 manganese, up to about0.15% carbon, up to about 0.5% aluminum, up to about 0.5 titanium, up toabout 0.015% sulfur and the balance essentially iron, which ischaracterized by exhibiting high resistance to cyclic oxidationresulting from balancing the constituents of the alloy within the rangesstated according to the following formula:

11789986 (percent Ce)1085 (percent Si)49.78

(percent Cr) +3965 (percent Si percent Ce)l+304.5 (percent Cr percentCe) +4484 (percent Si percent Cr)[+'16.65 [percent .Mn/ (percentSi+percent Ce) 560 and good resistance to stress corrosion crackingresulting from balancing the constituents of the alloy within the rangesstated according to the following formula:

1156;-I109.1 (percent Ni) +2690 (percent Mn percent Ce) 5.97 (percent Mnpercent Ni 1 .57

(percent Ni x percent Cr) 3 5 8 .5 (percent Si 5 00 2. The alloy ofclaim 1 containing at least 0.01% cerium.

3. The alloy of claim 1 containing no more than 1% manganese.

4. The alloy of claim 1 containing from 18% to 22.5% nickel, from 19% to22% chromium, from about 0.6% to about 1.1% silicon and from 0.015% toabout 0.04% cerium.

5. The alloy of claim 2 wherein nickel is present in the amount of about20, the chromium is present in the amount of about 20% 6. The alloy ofclaim 3 wherein nickel is present in the amount of about 20%, andchromium is present in the amount of about 20% 7. The alloy of claim 1wherein the nickel is present in the amount of about 20%, chromium ispresent in the amount of about 20%, silicon is present in the amount ofat least about 0.6% and cerium is present in the amount of about 0.02%

8. The alloy of claim 1 wherein the ratio percent Ni/ Cr is at leastunit, the silicon content is at least 1% and the manganese content doesnot exceed 0.5%

9. A weldable, oxidation and corrosion-resistant alloy consistingessentially by weight, of from about 15% to about 23% nickel, from about17% to about 23% chromium, from about 0.3% to about 1.5% silicon, from asmall but effective amount of cerium up to about 0.05% to improveoxidation and corrosion resistance, up to about 2% manganese, up toabout 0.15 carbon, up to about 0.5 aluminum, up to about 0.5% titanium,up to about 0.015% sulfur and the balance essentially iron, which ischaracterized by exhibiting high resistance to cyclic oxidationresulting from balancing the constituents of the alloy within the rangesstated according to the following formula:

11789986 (percent Ce) 1085 (-percent Si) 49.78

(percent Cr) +3965 (percent Si percent Ce) +3045 (percent Cr percentCe)+44.84 (percent Si percent Cr) 16.65 [percent Mn/ (percent Si+percentCe) 560 10. The alloy of claim 9 containing at least 0.01% cerium.

References Cited UNITED STATES PATENTS 2,174,919 10/1939 Kay -128 E2,683,663 7/1954 Tisdale 75-128 R 2,73 8,267 3/1956 Pakkala 75--1242,687,954 8/1954 Lohr 75-128 E 3,362,813 1/1968 Zolkowski 75124 HY LANDBIZOT, Primary Examiner U.S. Cl. X.R. 75124, 128 T

